A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a known lithographic apparatus, the drive unit of the positioning device for the substrate table may include two linear Y-motors each of which includes a stator extending parallel to the Y-direction and secured to a base of the positioning device, and a translator (Y-slider) movable along the stator. The base may be secured to the frame of the lithographic device. The drive unit may further include a linear X-motor that includes a stator extending parallel to the X-direction and a translator (X-slider) which can be moved along the stator. The stator of the X-motor may be mounted on an X-beam that is secured, near its respective ends, to the translators (Y-sliders) of the linear Y-motors. The arrangement is therefore H-shaped, with the two Y-motors forming the uprights and the X-motor forming the cross-piece, and this arrangement is often referred to as an H-drive.
The driven object, in this case the substrate table, may be provided with a so-called air foot. The air foot includes a gas bearing that is configured to guide the substrate table so as to be movable over a guide surface of the base extending at right angles to the Z-direction.
In a lithographic apparatus, reactions on the machine frame to acceleration forces used to position the patterning device (reticle) and substrate (wafer) to nanometer accuracies are a cause of vibration, impairing the accuracy of the apparatus. To minimize the effects of vibrations, it is possible to provide an isolated metrology frame, on which all position sensing devices are mounted, and to channel all reaction forces to a so-called force or reaction frame that is separated from the remainder of the apparatus.
In an alternative arrangement, the reaction to the driving force is channeled to a balance mass, which is normally heavier than the driven mass which is free to move relative to the remainder of the apparatus. The reaction force is spent in accelerating the balance mass and does not significantly affect the remainder of the apparatus. Balance masses moveable in three degrees of freedom in a plane are described in WO 98/40791, WO 98/28665 and U.S. Pat. No. 5,815,246.
FIG. 6 shows a cross section of a prior art X-motor configuration. The X-motor includes X-beam 107, a stator including a first stator part 105a and a second stator part 105b each mounted on the X-beam 107 and a translator 106 which can translate along the stator 105 and X-beam 107 in the X-direction, which is perpendicular to the plane of the drawing. An air bearing 108 is provided between the X-beam 107 and the translator 106 in order to provide an air cushion between the X-beam and the translator. The air bearing is pre-tensioned with the attraction force between the stator 105 and a motor part 109 of the translator 106.
When the X-beam 107 is accelerated in the y-direction by Y motors, the air bearing 108 pushes the translator in the y-direction. Due to the presence of the air bearing 108, mechanical contact between X-beam 107 and the translator 106 is avoided, so that the translator may slide in the X-direction.
However, due to increasing demands on acceleration of the substrate stage, the force and/or torque to be handled by the air bearing also increases. However, the maximum bearing surface of the air bearing is limited due to the dimensions of the X-beam. To deal with these higher forces and/or torques, the dimensions of the X-beam, for instance the height, could be increased to make incorporation of a larger air bearing surface possible. However, increase of the height of the X-beam would have a considerable impact on the further system design, and is therefore not desirable.